1. Field of the Invention
The present invention relates to a T.T.L. (through the lens) focus detection system utilizing an auxiliary light to help focus detection and a lighting device for projecting such an auxiliary light.
2. Description of the Prior Art
There have been proposed a variety of T.T.L. focus detection systems which detect focusing condition of an obective lens based on the light which has passed through the objective lens. The typical systems are the phase difference detection system disclosed in Japanese Patent Laid Open Publication Nos. 52-95221 and 54-159259 or contrast detection system disclosed in Japanese Patent Laid Open Publication No. 55-155308. In general, a T.T.L. focus detection system has such a disadvantage that it can hardly or cannot carry out focus detection when the object is dark or the contrast of the object is low. In such a case, a lighting device which projects an auxiliary light toward the object is used in order to help the focus detection. For example, such examples are disclosed in the specifications of U.S. Pat. No. 4,150,888 and in Japanese Patent Laid Open Publication No. 57-73709.
However, various problems are left unsolved in putting into practical use such focus detection system utilizing an auxiliary light emitted from a lighting device as described above. One of the most serious problems among them is how how to effectively light the object scene within the range from the near to far distance. Particularly, a lens exchangeable camera like a single-lens reflex camera which allows interchange of a variety of lenses is required to have a high focus detection accuracy and also to be capable of carrying out focus detection with respect to an object within a wider range of near to far distance with a variety of lenses from super wide-angle to super telephoto lenses in use.
Therefore, a lighting device used for such a lens exchangeable camera is also requried to be capable of lighting wider distance range from the near to far distance while covering the focus detection area spreading at a solid angle dependent on the angle of the field of a lens attached to the camera. In case the lighting distance is to be increased, it is possible to raise the light condensing capability of a light projecting optical system to make the spread of the projected light small However, in this case, the lighting range is narrowed and it is no longer possible to light the objects within a range from the near to far distances at the same time unless the light is projected from the camera along the optical axis through the objective lens. On the contrary, if the light condensing capability of the light projecting optical system is lowered and the spread of the projected light is widened, the lighting range can be widened but lighting distance is shortened. Moreover, if a power of light source of the light projecting optical system is increased, the lighting range can be widened and the lighting distance can also be elongated. However, this is not desirable in general because a light source having a large capacity is required and power consumption becomes large.
The lighting device disclosed in the specification of U.S. Pat. No. 4,150,888 is provided with a light projecting optical system which is pivoted in conjunction with focusing movement of an objective lens so that the angle of the light projection axis relative to the optical axis of the objective lens changes. With such a construction, objects within a range of near to far distance can be lighted in accordance with the focusing movement of the objective lens and the lighting distance can be extended by raising the condensing capability of the light projecting optical system However, the lighting range is narrowed and, if the objective lens is largely deviated from a proper focus position with respect to a main object, the main object cannot be lighted by the light, resulting in failure of focus detection. In addition, as the light projecting optical system should be pivoted as described above, a mechanical interlocking system which interlocks the light projecting optical system with the focusing movement of the objective lens is essential. Therefore, the lighting device is complicated in construction. Moreover it is difficult to provide a lens interchangeable camera with such an interlocking mechanism.
On the other hand, in the case of the lighting device disclosed in Japanese Patent Laid Open Publication No. 57-73709, a light projecting optical system is provided on the lens barrel of an exchangeable lens and this optical system is pivoted in conjunction with the focusing movement of the objective lens of the exchangeable lens. Such a lighting device is free from the disadvantage of difficulty in application to a lens exchangeable camera but still suffers from the other disadvantages of the lighting device disclosed in the specification of U.S. Pat. No. 4,150,888.
Moreover, such a lighting device suffers from another disadvantage that each exchangeable lens becomes very expensive since the light projecting optical system must be provided on the lens barrel of each excahngeable lens.